A continuously variable transmission (CVT) is capable of continuous drive train speed ratio changes. A vehicle utilizing a CVT operates with improved performance as compared to a conventional engine having a stepped transmission. CVT systems have become widely accepted, particularly in utility and work vehicles, such as tractors and the like, wherein vehicle speed must be matched to relatively large and varying load conditions.
One type of CVT design is a hydro-mechanical stepless drive system. It consists of a front side shuttle, compound planetary gear, and four mechanical ranges. The engine drives an input sun gear and the hydro-motor drives the ring gear as a variator. The dual outputs from the carrier and second sun gear are combined at a pinion shaft. In front of the planetary system is the shuttle arrangement with forward and reverse frictional clutches, and operatively configured after the planetary system are the four mechanical ranges shifting between the two planetary outputs. These shifts are carried out with frictional disk clutches at synchronized conditions. Thus, the requirement for electronic control is greatly simplified in this type of CVT design.
Another type of conventional CVT uses a belt or chain drive variator consisting of a belt or chain running between two variable diameter pulleys. Each pulley has a movable disc and an opposed fixed disc, with the discs defining sloped surfaces. The discs move closer or further apart to vary their respective diameters and, thus, provide an infinite number of transmission ratios (“speed ratios”). The discs are typically controlled by a pressure system (e.g., a hydraulic actuating system).
For a vehicle operating with a CVT, the vehicle speed is the product of engine speed and the CVT speed ratio, which is controlled by a CVT logic controller. When the operator sets a desired vehicle speed, the CVT logic controller calculates a corresponding engine speed and CVT speed ratio. Under most varying working conditions (e.g., varying loads and temperature changes), the difference between the desired and true CVT speed ratio can be controlled to be very close, and can be so small and stable that it is hardly noticeable in vehicle performance.
On the other hand, the difference between desired engine speed and true engine speed may be quite noticeable and constantly changing with the varying working conditions. For most operations, engine speed is controlled by a governor in accordance with a design droop line. When load (engine torque) increases, the engine speed (rpm) reduces along the droop line until the max torque curve is reached, at which point the engine speed decreases as a function of the design torque curve. As the load decreases, the engine speed recovers along the torque curve and then along the speed droop line. The engine speed is controlled at the set target value only when there is no load on the engine. Thus, engine speed will fluctuate with constantly changing vehicle working conditions under normal operations and, even though the CVT speed ratio can be closely controlled, the vehicle speed will change with the changing engine speed and deviate from the target vehicle speed (“speed lag”).
In at least certain conventional CVT control logic schemes, the vehicle speed lag is compensated for by detecting the engine speed droop. The control system measures the deviation between detected engine speed and target engine speed and commands the engine speed to adjust accordingly to compensate for the deviation. However, control theory dictates that a steady, errorless, operating state cannot be reached when the measured variable is also the control variable. Also, engine speed is very sensitive to environmental changes. When the engine load changes, it takes time for engine speed to settle at a new speed along the speed droop curve. During this time, however, the CVT control logic is attempting to compensate for the load induced speed change and the situation can occur wherein the load induced speed change and the commanded speed change adjustment are opposed, resulting in a fluctuating and unstable speed governing condition.
Thus, an improved control method and corresponding system for CVT vehicle speed lag compensation is desired.